The practical applications of CDMA systems show that the system capacity is not only limited by the reverse link capacity but also by the forward link capacity, which raises higher requirements of forward link power control (FLPC). It is the object of FLPC to assign reasonable power to forward traffic channels, and to minimize the interference with other users in the same cell and with the users in adjacent cells on the condition of ensuring the communication quality. Namely, the forward channel transmitting power should be as lower as possible on the condition that minimal required signal-to-noise ratio for demodulation in mobile stations is met. The adjustment of forward power control not only eliminates the “distance” effect, but also reduces the forward transmitting power to a minimum, depresses the interference with other users and increases the forward link capacity with communication quality ensured.
In IS-95 systems, FLPC tends to make every traffic channel transmit the lowest power under the condition that the desired frame error rate (FER) demanded by a mobile station is obtained. The mobile station continuously measures the FER in forward traffic channels and reports the power measurement report message up to the base station at a certain interval or at the time when the FER reaches a given threshold. Based on the FER report, the base station increases or decreases the transmitting power in the forward traffic channel with appropriate means. Of course, the base station limits the dynamic range of transmitting power in every traffic channel to guarantee the power to be under a maximum for not generating stronger interference and to be above a minimum for ensuring communication quality.
For RC1, the base station adjusts the transmitting power in forward channel based on the power measurement report message (PMRM) from the mobile station. The threshold report mode is used in IS-95 systems. In essence, based on the power threshold report, the quality of the current frame is determined indirectly, and the increase or decrease of power is decided thereby.
For RC2, besides by using PMRM, the base station adjusts the forward channel transmitting power by the erasure indication bit (EIB) in every reverse traffic frame via the codes received from the mobile station (EIB indicates whether the mobile station has received the last forward traffic data frame correctly). Since the reception of EIB is performed in every frame, it is obvious that the control period for adjustment of forward channel power by EIB is at least 20 ms.
It is seen that forward power control in IS-95A systems is a type of slow control mode with the control rate not higher than 50 Hz.
In CDMA2000-1X systems, when a mobile station enters a fast Rayleigh fading area, especially in the case when a high speed data traffic channel (SCH) works concurrently in the forward channel, the previous slow FLPC will no longer meet the requirements. Compared to CDMA95A systems, forward power control in CDMA2000-1X systems, on the one hand, is compatible with the forward power control mode of CDMA95A systems for RC1 and RC2, and on the other hand, introduces fast forward link power control (FFLPC) into forward links for RC3˜RC5 conditions. In IMT2000 standard, the fast closed-loop forward link power control mode at the adjustment speed of 800 Hz, 400 Hz and 200 Hz for RC3˜RC5 conditions is introduced, in which the outer loop power control on the mobile station side and the inner loop power control by both the mobile station and base station can be described as follows. (1) Outer loop power control. The target FER is obtained at a period of 20 ms by estimating and adjusting the setpoint based on Eb/Nt of the specified forward traffic channels. The adjustment of the setpoint can help the base station to obtain the appropriate transmitting level in the forward traffic channels of inner loop power control. There are three forms of setpoint: initial setpoint, maximum setpoint and minimum setpoint, which are sent to the mobile station by the base station in the form of a message. (2) Inner loop power control. In very power control group (PCG), the instruction of increasing or decreasing forward power control bit sent to the base station in reverse power control sub-channels of the current PCG is determined by comparing the estimated Eb/Nt of the received signal in the forward traffic channel with the current setpoint for the outer power control. The highest adjustment speed of power control instructions can reach 800 Hz at most.
Power control during soft handoff will be discussed in detail as follows.
In CDMA95A systems, the mobile station reports the conditions in forward links up to the selector vocoder module (SVM) of the base station controller (BSC) through the base stations involved in soft handoff. The output results calculated with power control algorithm stored in the SVM are sent to all base stations involved in soft handoff at the same time, so that all base stations transmit the same forward transmitting power. There does not exist synchronization operation of forward link transmitting power during soft handoff in IS-95 systems due to this centralized power control.
But in CDMA2000-1X systems, FFLPC is introduced for RC3˜RC5. Forward transmitting power of all base stations involved in soft handoff is independent of each other, i.e. they can be controlled by their own FFLPC. As shown in FIG. 1, if no special measure is taken during soft handoff, the system performance will degrade to a great extent during soft handoff.
Suppose that a mobile station is brought into soft handoff with 2 base stations involved and no special measure is taken for processing forward power, where the mobile station communicates with BS2 first and then is switched to BS1. It is assumed that both base stations transmit at a higher power level (e.g., approximately 5% of the total power of one base station). When the mobile station moves towards to BS1, the link between the mobile station and BS2 fades rapidly. At this time, the mobile station will most likely send instructions of decreasing transmitting power to BS1 if the link between the mobile station and BS1 is very strong. Due to the influence of distance, shadow and fading, the reverse link of BS2 is so weak that higher error rate occurs in the power control information received by BS2 in the reverse link. Therefore, the transmitting power by BS2 may be increased and maintained at about 9%˜10% of the total power of the base station. In the meanwhile, the link between the mobile station and BS1 is getting better and better, and the transmitting power of BS1 will gradually be decreased to about 1%˜2% of the total power of the base station. The above situation is possible when the power control bit error rate in BS2 reaches 50%. BS1 may correctly receive the instruction of decreasing power from the mobile station, but the power control bit error rate received by BS2 is relatively high due to the faded link between the mobile station and BS2, which results in a slower and more inaccurate power adjustment of BS2 than that of BS1. The pointless transmission of BS2 at higher power level not only results in the loss of system capacity, but also enlarges the multipath noise in the system. In the example mentioned above, the system capacity is decreased since transmission of BS2 is at a higher power level far beyond the need. About 30% of total power of the base station is assigned to the pilot channel, sync channel and paging channel, and the remaining 70% is assigned to traffic channels. If 10% of this 70% is used to maintain a link with poor quality, the system capacity is bound to decrease. Of course, BS2 may possibly increase the transmitting power by mistake and may possibly decrease the forward transmitting power by mistake since the reverse link between BS2 and the mobile station has greatly faded and the power control bit error rate received by BS2 is relatively high. This situation may lead the case that the transmitting power of BS2 is too low to maintain this link and the link will be dropped at last. Although the link between BS2 and the mobile station is of poor quality, the link may be usable when the FER rises again, for example, when the mobile station moves back to BS2. So it is required to maintain the link between the mobile station and BS2 at an appropriate transmitting power level.
The above discussions demonstrate that, if no special measure is taken after that forward links of RC3˜RC5 in CDMA2000-1X systems are involved in soft handoff, relatively big deviations of forward transmitting power between links involved in soft handoff may occur. Therefore, the forward link corresponding to a reverse link with poor reception quality will transmit high power unreasonably, so that the forward capacity of the system decreases and the severe interference in communications among other users within the system occurs; or the forward link transmitting power corresponding to the reverse link of poor reception quality is too low to maintain the link, decreases the stability of handoff process and the success rate of handoff. All those happened are due to that the FFLPC of RC3˜RC5 introduced in CDMA2000-1X systems is a type of distributed control (i.e. the individual control by each base station). Therefore, there are large deviations between reverse links of all base stations involved during soft handoff. That is to say, the signal-to-noise ratio (SNR) of base stations in the reverse pilot channels differs from each other, which results in different forward power control bit error rate located in the reverse pilot channels and finally results in greater deviations of transmitting power between forward links.
In fact, it is a usual case that one reverse link is strong and the remaining reverse links are week in practical working CDMA systems. The empirical data show that the possibility of the occurrence of the case is over 90%. Therefore, some special “Synchronization” measure must be taken to avoid the great degradation of system performance.
For the convenience of descriptions and understanding in the following contexts, the “synchronization” in the present invention is interpreted as follows. In CDMA2000-1X systems, when the mobile users in forward traffic channels with wireless setup RC3 and above are involved in soft handoff conditions, the forward transmitting power of base stations with poor reception quality in their reverse links (named as week reverse links) are forced to approach to the forward transmitting power of the base station with best reception quality in its reverse link (named as strong reverse link) after big deviations of transmitting power between all forward links appear. This process is called as “Synchronization”
A criterion is required for determining the reception quality of a reverse link. In fact, the frame error rate (FER) and Eb/Nt of a reverse link both indicate the reception quality of the reverse links of a base station to some extent. In principal, the higher the reverse link FER, the poorer the reception quality of the reverse link; the lower the reverse link FER, the better the reception quality of the reverse link; the lower the reverse link Eb/Nt, the poorer the reception quality of the reverse link; the higher the reverse link Eb/Nt, the better the reverse link reception quality of the reverse link.
Moreover, the reception quality of a reverse link represents directly the forward link power control bit error rate in the reverse pilot channel. Namely, the better the reception quality of a reverse link, the lower the power control bit error rate of the forward link; the poorer the reception quality of a reverse link, the higher the power control bit error rate of the forward link.
U.S. Pat. No. 6,154,659 and EP1047207A2 describe their own methods for balancing the forward link transmitting power during soft handoff respectively.
U.S. Pat. No. 6,154,659 describes a method for synchronizing the transmitting power of base stations (not limited to 2 base stations) during soft handoff by monitoring the reverse link FER via the BSC. The synchronization is implemented by adjusting the gains of base stations with poorer quality and the gains of base stations with higher quality. In the synchronization process, the BSC monitors the reverse link FER of base stations involved in soft handoff in a predefined detection period which is equal to 20 frames or a multiple of 20 frames. The reverse link FER reflects the FFLPC bit error rate of the base station to some extent. It is believed generally that the higher the reverse link FER, the higher the FFLPC bit error rate.
The process of synchronizing the transmitting power of base stations during soft handoff by monitoring the reverse link FER is described as follows.
It is assumed that all base stations involved in soft handoff are controlled by the same BSC. All base stations continually calculate their reverse link average FER in every detection period, and send their own average FER up to the BSC at the end of the period. The BSC compares these average FER to a predefined threshold, which can be either the average FER of the current reverse links with the best quality or an FER predefined as a parameter. Once the average FER of a base station exceeds the threshold, the BSC will adjust the transmitting power of the base station and set the transmitting power of the base station to be equal to the power of the base station with the lowest average FER in the last detection period. Thus, the power synchronization of the base stations involved in soft handoff during the handoff process is implemented, and the power of a base station is not decreased too much so that traffic links are dropped. In the meanwhile, the power of a base station will not be adjusted too high in synchronization due to incorrect reception of power control bit, which may result in very strong interference and a decreased system capacity. It is not preferable to keep very big differences of power between base stations with the best and worst links in order to guarantee that all base stations involved in soft handoff transmit the appropriate power and the traffic links connected to a mobile station are preserved till the mobile station withdraws from the soft handoff completely.
In the method of U.S. Pat. No. 6,154,659, the real time performance is poor in power synchronization of all base stations by the BSC because the minimum detection period of 20 frames (20*20 ms=400 ms) is used for detecting the reverse link FER. Furthermore, the estimation of the reverse link FER has to be based on the average estimate value in a period of certain time span. In general, the longer the period, the closer the estimated average FER is to the actual reverse link quality. But in view of the real time performance of synchronization, the period can not be too long. Moreover, a filter must be introduced for estimating FER (by using weightings to historical data) even with the minimum detection period of 20 frames (20*20 ms=400 ms). The calculation with such a filter function involved will not only consume the CPU resources (the buffers for historical and intermediate data, etc.), but also decrease the efficiency of the overall estimation algorithm. Compared to the synchronization methods of Patent EP1047207A2 and the present invention, the detection period of 20 frames results in poorer real time performance.
Patent EP1047207A2 describes a method for synchronizing the transmitting power of base stations involved in soft handoff (not limited to 2 base stations) by monitoring the reverse link Eb/Nt via the BSC. The synchronization is implemented by adjusting gains of base stations with poorer quality in reverse links and the gains of base stations with higher signal quality in reverse links.
The process for synchronizing transmitting power of base stations involved in soft handoff by monitoring the Eb/Nt in reverse links is described as follows.
All base stations involved in soft handoff demodulate continually the forward power control bits in the reverse links in every PCG period and adjust the transmitting power in forward traffic channels based on the power control bits, that is to say that FFLPC is performed. In the meanwhile, the average Eb/Nt is calculated in a detection period of 20 ms (1 frame=16PCG=20 ms); via a message, the base stations involved in soft handoff send the average Eb/Nt in the detection period and the current transmitting power in forward traffic channels to the BSC controlling the base stations; based on the comparison of average Eb/Nt in the detection period reported from base stations, the BSC determines which base station is the best in receiving forward power control bit data and defines it as BTS(x); by comparing the transmitting power of other base stations with that of BTS(x), the BSC put those base stations into the power synchronization queue if the deviations of transmitting power of the base stations exceed a predetermined threshold. The transmitting power of the base stations in the queue are then adjusted to be equal to that of BTS(x). The procedure will be performed repeatedly till the mobile station withdraws from soft handoff completely.
In this method, although the real time performance is improved in synchronizing all base stations by the BSC due to the minimum Eb/Nt detection period of 1 frame (1*20 ms=20 ms) in reverse links, but the calculations of Eb/Nt of reverse links are rather complicated. The system operation efficiency will still be affected because that many formulas (including difficult logarithmic operations) and tables have to be dealt with in the calculations, and the calculations are complex and time-consuming.